Polypropylene molding compositions containing low or medium flow polypropylene have good impact resistance when the composition includes a high molecular weight styrenic block copolymer having the structure polystyrene-hydrogenated polybutadiene-polystyrene, e.g. KRATON.RTM. G1650 thermoplastic elastomer. The impact strengths are suitable for use of the molding compositions in large automotive parts. However, research has established that polypropylene molding compositions for large automotive parts must also have improvements in rigidity, heat deformation resistance, low temperature impact resistance, and surface appearance.
High flow polypropylene molding compositions that are suitable for large automotive parts except for inferior low temperature impact strength and inferior high temperature heat deformation are produced by blending low molecular weight polystyrene with low molecular weight diblock copolymers of polystyrene-hydrogenated butadiene. A blend of the triblock and diblock copolymers is commercially available as KRATON.RTM. G1657 elastomer which is available from Shell.
U.S. Pat. No. 5,045,589 describes much improved polypropylene molding compositions comprising a crystalline ethylene/propylene block copolymer, an amorphous polypropylene copolymer, and a thermoplastic elastomer which can be a polystyrene-hydrogenated polybutadiene-polystyrene elastomer, e.g. KRATON.RTM. G1650 elastomer, or a polystyrene-hydrogenated polyisoprene-polystyrene elastomer.